The present invention relates to coated particles and a method for their preparation. The present invention further relates to thermally reactive powders used in flame spraying processes.
Thermally reactive powders are used to deposit adhesive films, coatings with superior properties (including wear resistant, corrosion resistant and electrical resistant), as well as the manufacture of monolithic products, for example, by the method of self-propagating high temperature synthesis (SHS).
The intense heat generated during the thermally reactive process accelerates the rate of the redox reaction between the components of the composite powder (for example, between aluminum and nickel or iron). Moreover, the reaction can either take place in the whole volume of the powder or spread from one part of the volume to another.
As a result of the reaction, depending on the contents of the gaseous phase, intermetallics, oxides or other compounds are formed. The reaction can take place either in the liquid or the gas phase. Composite powders made by this process have an unusual range of properties and are unique in their strength, ductility and resistance to oxidation over a broad range of temperatures.
The close proximity of the two metal species to one another is important to achieving a smooth continuous reaction. One way of obtaining the close contact of the two materials is to coat one with the other.
U.S. Pat. Nos. 3,338,699 and 3,436,248 disclose metal-coated metals prepared by coating the core metal with a paint composed of an organic binder and powders of the second metal. However, the coating does not adhere well and impurities (decomposition products for the organic binder) are introduced into the powder during the thermal reaction.
Coating a core metal with salt solution of the second metal followed by thermal decomposition of the metal salt has been used to obtain metal-coated metals. Decomposition of the deposited metal salt results in gas evolution and precipitate formation, thus compromising the quality of the metal coating. Degradation of the metal salt layer in the presence of hydrogen leads to cleaner decomposition products, however, impurities still remain.
It is an object of the present invention to provide a method for preparing particles with a variety of coatings. It is a further object of the present invention to prepare thermally reactive powders in the form of metal-coated metals. It is a further object of the invention to prepare such powders free of impurities and additives with optimal adhesion between the metal coating and metal core.